automate git subtree-push pull requests

[ytmimi]

r? @calebcartwright

Here's an example of what the automated subtree-push PR looks like, and Here's a link to the GitHub Actions logs.

I want to note that PRs are still created even if there are issues creating the merge commit. In those cases one of us would just need to manually fix the merge conflicts and bump the toolchain. Here's an example of what the PR might look like when the merge fails for any reason, and here are the associated logs. I believe this one failed because of git history issues that I fixed 😁.

I've got this setup as a manual trigger right now. I figure it would be best to run it manually a few times and try to work out any kinks, and if things are working well we can consider running this on a cron schedule.